1. Field of the Invention
This invention relates to methods for forming dry, high bulk, friction materials into rigid structures which are substantially immediately useable in applications requiring friction resistant structures without further modification. More particularly, this invention relates to an improved method of producing substantially rigid, friction material structures in the form of brake shoes, disk brake pads, brake blocks and the like.
2. Description of the Prior Art
Initially, it is to be understood that the terms "friction material", "friction resistant material" and "energy dissipating material" are all used herein synonymously and interchangeably to denote those materials which, when formed into solid bodies, may be used in brakes and clutches for dissipating mechanical kinetic energy by frictional contact with another material, such as in uses where contact between the two materials takes place with a structure formed of one material in motion while the other structure is at rest and in applications where the friction material is slowly worn away by such contact. Friction materials include those materials employed for manufacture of brake shoes used in drum type brakes in automobiles, brake pads used in disk type brakes in automobiles, brake blocks used in brake assemblies of trucks, buses and other heavy vehicles, and brake blocks forming a part of brake assemblies used on railroad locomotives and railroad cars. Furthermore, friction material as used herein also encompasses those materials used for the manufacture of friction plates in clutches.
Also as used herein, it is to be understood that the terms "brake linings", "disk brake pads", and "brake blocks" mean molded bodies of friction resistant material which have been formed under heat and pressure into substantially rigid, solid structures which may be assembled into a brake system with a minimum of further machining after the application of heat and pressure. Disk brakes are rapidly replacing drum type brakes as standard equipment on passenger cars. Many automobiles now have disk brakes on the front wheels and an increasing number of automobiles have disk brakes both front and rear. Disk brakes are generally recognized to be better than drum type brakes. Disk brakes rapidly dissipate the heat which builds up during brake application due to the frictional contact between the brake pad, manufactured of friction material, and the moving surface against which the brake pad is applied. Disk brakes dissipate heat faster than do conventional drum type brakes, thereby reducing the heat build-up in the brake material which can produce brake fade. Brake fade is the phenomenon whereby resistance to motion produced when friction material is pressed against a moving surface is reduced due to the friction material becoming slightly fluidized and, therefore, lubricated. Since heat builds up on every application of brakes, it is desirable to dissipate the heat rapidly, thereby permitting more frequent application of a vehicle's brakes without danger. Thus, an automobile or other vehicle equipped with disk type brakes can be stopped more quickly and more often than an automobile equipped with conventional drum type brakes.
A second major advantage of disk brakes over drum brakes is that disk brakes dry more quickly than drum type brakes after exposure to the lubricating effects of water.
With increasingly stringent federal safety standards, an increased demand for disk brakes in motor vehicles is expected.
Disk brake assemblies and disk brake friction material pads for insertion therein have a substantially different configuration than do drum brake friction material inserts. Because of this, it has not proven feasible to simply modify the present highly developed methods, techniques and procedures for the manufacture of friction material inserts for drum brakes to the efficient manufacture of disk brake pads. The technology for manufacture of drum brake friction material inserts has developed to a highly efficient, productive, substantially continuous procedure, which typically involves extrusion and impregnation of tape-type material with the friction material, whereupon the combination is molded or otherwise formed into the drum brake lining or drum brake friction material insert.
The configuration requirements of truck brake blocks and railroad brake shoes and the density of friction material required therefor have also required that noncontinuous methods of manufacture such as molding be utilized. These methods are substantially the same as those used for the manufacture of automobile disk brake friction material inserts or pads.
Presses for molding friction materials under predetermined heat and pressure conditions have been used commercially since at least 1965. For example, since 1965 Raybestos Corporation has used molding and feeding apparatus substantially as disclosed in U.S. Pat. Nos. 3,278,992 and 3,225,963 for feeding, heating, forming precharges and molding friction materials into finished rigid friction material structures. U.S. Pat. No. 3,278,992 of W. Strauss, entitled "Molding Apparatus" granted Oct. 18, 1966 and assigned to Pennwalt Corporation is sometimes referred to hereinafter as the "3,278,992 patent" or similarly. U.S. Pat. No. 3,225,963 of V. F. Arpajian, entitled "Hopper Apparatus and Method", granted Dec. 28, 1965 and assigned to Pennwalt Corporation, is sometimes referred to hereinafter as the "3,225,963 patent" or similarly. The disclosures of the 3,278,992 and 3,225,963 patents are hereby incorporated herein by reference.
Prior to such use of molding apparatus and associated hopper apparatus for forming friction materials into rigid bodies, the earlier process by which rigid friction material bodies were formed comprised the steps described immediately below.
Initially, according to prior practice, batches of friction materials were mixed according to known recipes in closed mixers, and then discharged either continuously or in bulk from the mixers into closed containers. A typical recipe for friction materials included substantial percentages of asbestos (40 to 70%) and binder material (10 to 30%), along with smaller percentages (up to 50%) of various metallic oxides and the like to provide reasonable life for the finished friction material product. A summary of the problems involved in formulating an appropriate friction material recipe is contained in the paper Automotive Brake Lining Materials by A. J. Carter presented at the annual meeting of the Society of Automotive Engineers in January of 1950. The disclosure of that paper is incorporated herein by reference.
The second step of the process as known prior to 1965 was to transfer closed containers of the mixed friction material by conveyor or truck from the mixer to a preformer.
The next step was to feed the friction material in the mixed state into a preformer. Typically, closed containers of mixed friction material would be fed by hand from the top of a preform machine into the preformer. This step of the process necessarily involved human contact with the brake material which was undesirable due to the health hazards involved in handling asbestos. Additionally, during this dusty operation the individuals feeding the preformer were exposed to the risk of inhaling or ingesting asbestos dust.
Due to the lofty nature of the friction material mixture, and the asbestos therein in particular, it was previously necessary to compress a relatively large volume of the mixture into a solid form. The solid form was next subdivided by trimming into so called cold preforms. Each preform was sized to fit into a mold cavity. The disadvantage of this prior practice is that asbestos dust was released into the shop atmosphere during the steps of compressing the solid form and trimming it into preforms.
The next step was to manually transfer the trimmed preforms in closed boxes from the trimming station to a molding press, thereby exposing personnel to the dust created by bumping the preforms about in transit.
The next step in the process was that of loading a molding press with the trimmed preforms.
The following step was to mold the preforms under pressure and heat. This step consolidated the loose asbestos fibers with the binder and produced a substantially dustless product.
The next step was that of transferring the molded rigid friction material structures from the molding press to a baking oven for post molding cure. Open conveyors or pallets were used for this transferring step.
Finally, the molded rigid friction material bodies were cured in a bake oven.